Sn): Phase

Dec 26, 2016 - Unfortunately, many new chalcogenides with record high SHG efficiencies are NPM, such as Ba3CsGa5Se10Cl2 exhibiting a maximal SHG ...
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Infrared SHG Materials CsM3Se6 (M = Ga/Sn, In/Sn): Phase Matchability Controlled by Dipole Moment of the Asymmetric Building Unit Hua Lin,‡ Ling Chen,*,† Ju-Song Yu,‡ Hong Chen,‡ and Li-Ming Wu*,† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, Beijing Normal University, Beijing 100875, People’s Republic of China ‡ Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *

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furthermore, by controlling the initial reactant concentration, the final products, PM [Hdpa]2NbOF5·2H2O or NPM HdpaNbOF4, can be synthetically controlled.12 Zyss reported that the optimal orientation of organic molecules is responsible for the bulk phase matching.13 Poeppelmeier also reported that inorganic NaVOF4(H2O) and NaVO2−xF2+x synthesized at different temperatures with the same starting reagents are PM or NPM, respectively.14 Unfortunately, many new chalcogenides with record high SHG efficiencies are NPM, such as Ba3CsGa5Se10Cl2 exhibiting a maximal SHG about 100 times that of benchmark AgGaS2 (100 × AGS),15 Ba23Ga8Sb2S38 (22 × AGS),16 and AM9Q12 (10−40 × AGS).17−19 Despite showing the desired excellent dij, these compounds could be unusable because of the nonphase matchability. To our best knowledge, only seven new PM chalcogenides have been reported recently, and their SHG and LIDTs are listed as follows: Na2BaSnS4 (0.5 × AGS @ 2090 nm; 5.1 × AGS @ 1064 nm),20 Na2ZnGe2S6 (0.9 × AGS @ 2090 nm; 6.0 × AGS @ 1064 nm),21 Na2Hg3Sn2S8 (2.8 × AGS @2090 nm; 1.0 × AGS @ 1064 nm),22 SnGa4Se7 (3.8 × AGS @ 2050 nm; 4.6 × AGS @ 1064 nm),23 PbGa2GeSe6 (4.3× AGS @ 2050 nm; 3.7 × AGS @ 1064 nm),24 BaGa2SnSe6 (5.2 × AGS @ 2050 nm; N/A),25 and [Rb3Br][Ga3PS8] (2.0 × AGS @ 1950 nm; 31.0 × AGS @ 2050 nm).26 However, the intrinsic reason for why they are PM remains unknown. Thus, to probe the origin of phase matchability is the second challenge of designing SHG materials. This study presents two new PM CsM3Se6 (M = 0.33 Ga (or In)/0.67 Sn) with excellent SHG efficiency (3.5 and 4.0 × AGS @ 2.05 μm) and high LIDT (10.0 and 9.2 × AgGaS2 @ 1064 nm), distinguishing themselves as one of a few best materials known to date. The calculated d15 (53.3, 63.9 pm/V) are consistent with the experimental observations. The calculated birefringence (Δn = 0.053, 0.071) agrees well with the observed phase matchability. In comparing with the NPM parent CsM9Se12,18 CsM3Se6 illustrates for the first time that dipole moment orientation and magnitude of the asymmetric building unit may determine the phase matchability.

econd-harmonic generation (SHG) occurs as a result of the part of the atomic response that scales quadratically with the applied optical field; thus, SHG crystals are able to convert the laser frequency to produce new coherent laser with tunable frequencies which are otherwise unavailable.1−4 Chalcogenides are superior SHG materials in the IR spectrum region,5 where the notable oxide SHG crystals, such as LiB3O56 and βBaB2O4,7 are not suitable because of their opacity. However, a few commercial IR SHG materials, i.e., AgGaS2 (AGS),8 AgGaSe2,9 and ZnGeP2,10 suffer drawbacks, such as lower laserinduced damage threshold (LIDT) or strong two photon absorption. Therefore, to develop new effective IR SHG materials is in urgent demand. Ideally, a nonlinear optical (NLO) material should possess simultaneously large secondorder nonlinear optical susceptibility (dij) and high LIDT, which are usually inversely correlated.11 Several effective strategies are proposed to obtain large dij in a noncentrosymmetric (NCS) material.12−19 We consider that to enhance LIDT should take the following three aspects into account: (1) to broaden the band gap (Eg) of a material in order to decrease the heat generated in situ by the laser light absorption process, but the wider the Eg, the smaller the dij will be;11 (2) to find material with intrinsically large thermal conductivity that can transport the heat well; and (3) to find material with high thermal stability. Thus, the first challenge of designing SHG materials is to realize a suitable balance between the large dij and the high LIDT within a single material. On the other hand, to be useful in real applications, the SHG materials must be phase matchable (PM); otherwise, a dramatic decrease in the efficiency of the SHG process occurs. Only under the phase matching condition (the wave vector mismatch between the fundamental and the second harmonic frequencies Δk = 0), the generated SHG light can be efficiently output.4 Generally, in order to achieve PM through the use of birefringence (Δn) of crystals, a moderate Δn is practically requested.4 However, to find out whether or not a NCS material is PM relies on mainly the routine experimental phase marching condition measurement on polycrystalline samples.12 This is mostly because the relationship between bulk phase matchability and microcrystal structure is not fully understood, although very few reports have been primarily discussed. For instance, Poeppelmeier indicated in hybrid, the arrangement of the organic molecule dictates the phase matchability; © XXXX American Chemical Society

Received: November 25, 2016 Revised: December 25, 2016 Published: December 26, 2016 A

DOI: 10.1021/acs.chemmater.6b05026 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

significantly, i.e., a loose layered moiety in CsM3Se6 constructed by one crystallographically independent M (Figure 1c) vs a condensed moiety in CsM9Se12 constituting three crystallographically independent M atoms (e.g., M1 = 0.44 Ga/0.56 Cd; M2 = 0.56 Ga/0.44 Cd; and M3 = 0.67 Ga/0.33 Cd) (Figure 1d). Nevertheless, CsM3Se6, CsM9Se12,18 and AgGaSe29 are all structurally related to the cubic ZnSe structure type, in which the Se atoms are cubic close packed (ccp) and the Zn atoms occupy half of the tetrahedral voids.27 The optical and thermal properties of CsM3Se6 (M = 0.33 Ga (or In)/0.67 Sn) were measured by employing solid-state diffuse reflectance, IR spectroscopies, and DSC-TG thermal analyses. The results indicate that polycrystalline CsM3Se6 possess semiconducting band gaps of 1.78 and 1.87 eV (Figure S4), wide transparent regions of 0.69−25 and 0.65−25 μm (Figure S5, these ranges are comparable with that of the benchmark AGS, 0.6−23 μm17−19), and excellent thermal stability up to 957 and 944 K (Figure S6). Significantly, CsM3Se6 demonstrates strong powder SHG efficiencies (3.5 and 4.0 times that of PM AGS) with phase-matching behavior at the incident 2.05 μm (Figure 2c) and high LIDTs (14.2, 13.0

Compounds CsM3Se6 (M is disordered by 0.33 Ga (or In) and 0.67 Sn) were synthesized with high yields by solid state reactions of the mixture of Ga (or In) (5N), Sn (5N), Se (5N), and reactive flux CsCl (3N). The optimal Cs/Ga(In)/Sn/Se ratio was established as 2:1.3:1.775:6 (Supporting Information, Figure S1). Single crystals with sizes up to 2 × 2 × 3 mm3 are obtained directly from the reactions (Figure 2a, b). The homogeneity of the product is checked by the powder XRD data (Figure S3), and the EDX data indicate the presence of Cs, Ga(In), Sn, and Se in a ratio of 1:1:2:6 that agrees well with the single crystal refinement (Table S2−4). CsM3Se6 are uniaxial crystals and crystallize in the NCS polar trigonal space group R3 (No. 146) with a = 10.518(8) Å, c = 9.539(2) Å, and Z = 3 for CsGaSn2Se6 (M is disordered by 0.33 Ga and 0.67 Sn), and a = 10.652(5) Å, c = 9.688(8) Å, and Z = 3 for CsInSn2Se6 (M is disordered by 0.33 In and 0.67 Sn). The crystallographically unique atoms are Cs (3a), M (9b, disordered by 0.33 Ga (or In) and 0.67 Sn), Se1 (9b), and Se2 (9b). The Cs atom resides at the 3a site centering a Se12 cuboctahedron with normal Cs− Se distances ranging from 3.8466(8) to 3.9642(7) Å (Table S4). The M atom is coordinated by four Se atoms in a distorted tetrahedral geometry with M−Se bond lengths of 2.4812(6)− 2.4949(8) Å for CsGaSn2Se6 and 2.5408(7)−2.5493(8) Å for CsInSn2Se6. As shown in Figure 1c, being operated by the

Figure 2. Single crystal photos of CsM3Se6 (a) CsGaSn2Se6 and (b) CsInSn2Se6. (c) Phase-matching curves of CsM3Se6 (particle size vs SHG intensity) at incident wavelength of 2.05 μm with the PM AGS as a reference. (d) The relative SHG intensities and LIDTs of CsM3Se6 and benchmark PM AGS in the particle size range of 150− 210 μm at the incident wavelength of 2.05 μm.

Figure 1. Crystal structures of (a) CsM3Se6 and (b) CsM9Se1218 showing the diamond-like packing of the MSe4 tetrahedral building unit alone the c axis. Green ball: Cs; yellow ball: Se. (c) In CsM3Se6, the trimeric MSe4 tetrahedra are arranged in a planar manner in the ab plane. (d) A single ab layer in CsM9Se12 consists MSe4 tetrahedra with crystallographically independent M centers: purple: M1; cyan: M2; blue: M3.

MW/cm2, that are estimated to be 10.0 and 9.2 times greater than that of benchmark AGS, 1.44 MW/cm2; Figure 2d, Table S6, and measurement details listed in the Support Information). These properties distinguish CsM3Se6 as one of a few best IR SHG materials known to date. The linear and nonlinear optical properties of CsM3Se6 are studied with the aid of the ab initio calculations performed by VASP software (Table S6, Figures S10−12). The band structures and densities of states (DOSs) are shown in Figure S9 and Figure 3a. In order to understand the origin of the SHG response of title compounds, the cutoff-energy dependent on the largest second-order tensor d15 is studied. Figure 3b shows that in the regions of VB-1 (dominated by Se 4p and Sn 5p orbitals) and CB-2 (dominated by Se 4p, Sn 5s, and Ga(In) ns/ np orbitals), the d15 values are the most sharply increased, which contribute mainly to the second order nonlinear susceptibility. The calculated d15 values of CsM3Se6 are 53.3

threefold rotation axis, the MSe4 tetrahedron is trimerized into an isolated supertriangle (marked by the red circle) that is arranged in a planar manner within the ab plane. Such supertriangles are joined along the c axis at each of the vertices (Se atoms) into a 3D network (Figure 1a). This structure is a loose variant of the parent compound CsM9Se12.18 These two structures differ mainly in their a parameters. As shown in Figure 1a,b, the c parameters in both structure types are about three times the height of the MSe4 tetrahedron defined roughly by the M−Se bond length. Such bond lengths vary only about 2% upon the M identity changing from Ga (or In)/Sn to Ga (or In)/Cd.18 However, the ab layers in both types differ B

DOI: 10.1021/acs.chemmater.6b05026 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 4. Plot of birefringence (Δn) as a function of magnitude of dipole moment (Debye: D) for the PM members of the BaM3Q6 family25,28 and CsM3Q6 (this work). The red lines represent the leastsquares fitting, and the goodness-of-fit is R2 = 0.9632.

(ordinary index). And the optic axis is the z axis, nz = ne (extraordinary index). As a result, the birefringence is defined as Δn = ne − n0 = nz − nx, so the Δn is determined by the difference between the refractive index along the z direction and that lying in the xy plane. We found that, in both types of structures, the MSe4 building units are distorted with different degrees, yet the difference is insignificant. The dipole moment of each asymmetric MSe4 tetrahedral building unit is noted as a vector P, of which the magnitude is defined by the distance between the M center to the negative charge center that is the geometric center of the four Se atoms. Similar to the chargetransfer axis in hybrid NLO materials,12 the P is able to indicate both degree and orientation of the distortion of the MSe4 building unit. As listed in Table S7 and shown in Figure S14, within the CsM3Se6 type family, both the Ba-variants (five known members to date)25,28 and the Cs-variants (up to now, only two known examples as reported herein) are PM compounds. All of them have only one P direction, for example, P = (−0.004, −0.011, 0.002) in CsGaSn 2 Se 6 (CsM3Se6 type) primarily oriented in the xy plane. Therefore, the magnitude of P reflects the structural anisotropism between the z axis and the xy plane. Such anisotropism reflects the difference between the nz and the nx (or ny), which defines the Δn value (Δn = nz − nx). Consequently, the CsM3Se6 type compounds have large Δn values and are observed to be PM. In addition, the dipole moment magnitude of the Ba-variants, BaM3Q6,25,28 nicely shows a linear correlation with the Δn with R2 = 0.96 (Figure 4). On the contrary, CsM9Se12 (M = 0.56Ga/0.44Cd) possesses three independent MSe4 building units, i.e., three dipole moments listed as follows: P1 (−0.004, −0.001, 0.011) roughly along the z direction; P2 (−0.008, −0.009, −0.001), nearly lying in the plane of xy; and P3 (0.002, −0.007, −0.008) along the direction that is with an angle of 45° to the z axis. These mean that dipole moments of the asymmetric building unit in CsM9Se12 do not have orientation preference; consequently, the CsM9Se12 type compounds are optically isotropic. As a result, the nz does not deviate significantly from the nx (or ny), and consequently the Δn values of CsM9Se12 type are small. Eventually, all CsM9Se12 type compounds are observed to be NPM. Clearly, the CsM3Se6 and CsM9Se12 type structures mainly differ in dipole moment orientation of the asymmetric building unit, which is considered to be responsible for their different phase matching behaviors. Moreover, our primary calculations also show that Δn is proportionally correlated to

Figure 3. (a) DOS of CsM3Se6 (the orbitals with minor contributions are omitted for clarity) and (b) static SHG coefficients of CsM3Se6 as a function of the cutoff energy. Dashed line, EF; dotted line, different regions in valence bands (VB) and conduction bands (CB).

for M = 0.33 Ga/0.67 Sn and 63.9 pm/V for M = 0.33 In/0.67 Sn, respectively. These values are 3.5 and 3.0 times larger than that of AgGaS2 (d36 = 18.2 pm/V) at the wavelength of 2.05 μm (i.e., 0.61 eV) and are in good accordance with the experimental measurements (Figure 2). A potential PM crystal shall have a moderate optical birefringence.4 If the birefringence is too small (e.g., Δn < 0.04), the PM equation will have no solution. If the birefringence is too large (e.g., Δn > 0.10), the walk-off effect limits the overlap of the ordinary and extraordinary rays and decreases the efficiency of the nonlinear mixing process.4,11 Our calculations reveal that the CsM3Se6 compounds have moderate birefringence (Δn) values (at the wavelength of 2.05 μm, 0.61 eV), such as CsGaSn2Se6, 0.05; and CsInSn2Se6, 0.07, both are larger than that of AgGaS2 (0.039). These Δn values fall in an optimal range of 0.04−0.10,11 indicating that CsM3Se6 easily achieves the phase-matching feature (Figure S13). Consistently, CsGaSn2Se6 and CsInSn2Se6 are observed to be PM (Figure 2), and more details are summered in Table S6, whereas the CsM9Se12 compounds have small Δn values that are only 14− 28% of those of the CsM3Se6, for instance, CsGa5Cd4Se12, 0.014, and CsIn5Cd4Se12, 0.01, indicating that their phase matching condition can be hardly realized. This also agrees with the experimental observations that the CsM9Se12 compounds are all NPM.18 Very interestingly, BaGa2SnSe625 is a derivative of the CsM3Se6, of which the Δn is calculated to be 0.13 agreeing with that proposed by Wu.25 (Figure 4) These results reveal that the CsM3Se6 structure is unique in terms of the birefringence (Δn) that can be turned significantly within the optimal range proposed for good IR phase matchability. As discussed in the structure section, the structures of CsM3Se6 and CsM9Se12 type18 are closely related, and both are diamond-like networks constructed by MSe4 tetrahedra. However, the former is PM and the latter is NPM. To understand such a difference, the structures and optical properties are carefully studied. Both CsM3Se6 and CsM9Se12 crystallize in the R3 space group and are uniaxial crystals, in which the x and y directions are crystallographically equivalent. Thus, the corresponding refractive indexes follow: nx = ny = n0 C

DOI: 10.1021/acs.chemmater.6b05026 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials the magnitude of P along each of the crystal axes and further work is ongoing. In conclusion, we report herein two new phase matchable IR SHG materials crystallizing in a polar noncentrosymmetric R3 space group, the uniaxial crystals of CsM3Se6 (M = 0.33 Ga (or In)/0.67 Sn). With careful studies on CsM3Se6 together with their chemically and symmetrically related CsM9Se12 ones as well as their Ba-variants, we reveal for the first time that dipole moment orientation and dipole moment magnitude of the asymmetric building unit dictate the phase matchability. Remarkably, these compounds also exhibit excellent NLO properties (3.5 and 4.0 × AgGaS2 @ 2.05 μm), which are understood by incorporation of the density functional theory calculations and cutoff-energy-dependent nonlinear optical coefficient analyses. In addition, title compounds show other merits as promising IR NLO materials, such as high LIDTs (10.0 and 9.2 × AgGaS2 @ 1064 nm), moderate optical birefringences, and broad IR transparencies. This work provides a new possibility for designing phase matching NLO materials via the adjustment of dipole moment of the asymmetric building unit that is accessible and controllable by chemical means.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05026. Details of experimental and theory results, together with additional figures and tables (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(L.C.) E-mail: [email protected]. Tel.: 86-010-62209380. *(L.-M.W.) E-mail: [email protected]. Tel.: 86-010-62209980. ORCID

Li-Ming Wu: 0000-0001-8390-2138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the NSF of China (Nos. 21225104, 21233009, 21301175, 91422303, 21571020, and 21171168). We thank Prof. Yong-Fan Zhang at Fuzhou University, China, for helping with the DFT calculations and Dr. Bing-Xuan Li and Prof. Ge Zhang at FJIRSM, CAS, for helping with the NLO measurements.

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REFERENCES

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DOI: 10.1021/acs.chemmater.6b05026 Chem. Mater. XXXX, XXX, XXX−XXX